1. Field of the Invention
The invention relates to cellular wireless communication that uses multiple antennas to achieve improved network performance.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
An LSAS (large-scale antenna system) communication network has a number of base stations distributed over a geographic region, where each base station has a multi-antenna array that is used to communicate with each of one or more terminals (e.g., wireless mobile units) located within the coverage area of the base station. The essence of LSAS is that large numbers of service antennas communicate with a smaller number of terminals all using the same time/frequency resources. Knowledge by the service antennas of the propagation channels to the terminals enables, on the downlink, the focusing of simultaneous data-bearing beams to the terminals and, on the uplink, the separation of the data-bearing transmissions that are simultaneously transmitted by the terminals. Adding more service antennas produces sharper downlink beams and better discrimination of the individual uplink transmissions. Each terminal has (i) a different uplink channel having unique uplink characteristics to each service antenna of a base station and (ii) a different downlink channel having unique downlink characteristics from that service antenna.
In order to successfully recover information transmitted from each of its terminals, each base station (post-)compensates its received uplink signals based on estimated uplink channel coefficients that characterize the different uplink channels through which the uplink signals have propagated from the different terminals to the base station's service antennas. In addition, in order to ensure that its terminals will be able to successfully recover information from its transmitted downlink signals, each base station (pre-)compensates its transmitted downlink signals based on estimated downlink channel coefficients that characterize the different downlink channels through with the downlink signals will propagate from the base station's service antennas to the different terminals.
Each uplink and downlink channel includes both (i) a wireless (i.e., over-the-air) portion corresponding to the air link between a base station service antenna and a terminal over which the uplink/downlink signals are transmitted and (ii) wired portions corresponding to the transceiver circuitries at the base station and terminal that process those uplink/downlink signals. The channel characteristics of the over-the-air portions of uplink/downlink channels typically change much faster than the channel characteristics of the wired portions of those channels. Furthermore, at any given time, the channel characteristics of the over-the-air portion of an uplink channel from a terminal to a base station service antenna are substantially identical to the channel characteristics of the over-the-air portion of the corresponding downlink channel from that same base station service antenna to that same terminal. However, the channel characteristics of the wired portion of that uplink channel can be substantially different from the channel characteristics of the wired portion of that corresponding downlink channel.
To generate estimated uplink channel coefficients, each terminal periodically (e.g., once every coherence interval) transmits a known pilot sequence that the base station uses to estimate the characteristics of the different uplink channels. The base station then uses those estimated uplink channel coefficients to post-process its received uplink channels. In order to generate estimated downlink channel coefficients to be used to pre-process its downlink signals, each base station modifies its uplink channel coefficients based on calibrated differences between (i) the uplink channel characteristics of its wired portion of an uplink channel and (ii) the downlink channel characteristics of its wired portion of the corresponding downlink channel.
Conventional techniques for calibrating the channel differences between a base station's uplink and downlink wired portions include using a special reference antenna built into the base station's antenna array to (i) transmit special uplink calibration signals to the base station's service antennas and/or (ii) receive special downlink calibrations signals from the base station's service antennas.
As LSAS antenna arrays get larger, the use of such special reference antennas becomes impractical. First of all, the time it takes to complete the calibration process is proportional to the number of service antennas. Furthermore, in order to simplify the decision of transmission power of the reference antenna, the reference antenna has to be placed in a position so that its horizontal distances to the different service antennas are approximately identical. During the calibration process, the reference antenna communicates with the service antennas over the air such that the LSAS system cannot serve the terminals at that time. Moreover, the placement constraint of the reference antenna makes it difficult for the reference antenna to be in the same enclosure used for the service antennas.